High-speed printing systems typically include one or more imaging units. Each imaging unit has one or more inkjet printheads and a controller controls each inkjet printhead to eject a fluid (such as ink or another composition) onto a receiving surface. Each inkjet printhead includes a nozzle plate that includes a plurality of orifices (nozzles) through which ink from inside the inkjet printhead may be controllably ejected.
An inkjet printhead typically includes a fluid chamber and one or more nozzles. Pressure inside of the fluid chamber is increased relative to ambient air pressure to force a drop of fluid through the nozzle(s). Some inkjet printheads use a piezoelectric element that deforms a wall of the fluid chamber to reduce the volume thereof and thereby increase the pressure within the fluid chamber. Alternately, a heating element may be used to vaporize some of the fluid (or a constituent of the fluid such as a fluid carrier or a solvent) in the fluid chamber to form a bubble therein, which increases the pressure inside the fluid chamber. A controller controls the current that is passed through the piezoelectric element to control the deformation thereof or to control the current through the heating element in turn to control the temperature thereof so that drops are formed when needed. Other types of inkjet technologies known in the art may be used in the printing systems described herein.
In a printing system, an inkjet printhead may be secured to a carrier and disposed such that the nozzles of the inkjet printhead are directed toward the receiving surface. The carrier may be manufactured from steel or other alloys that can be milled to a high precision. More than one inkjet printhead may be secured to the carrier in this fashion in a one or two-dimensional array. To form a printed image, the carrier and a medium to be printed on are moved relative to one another as drops of ink are controllably ejected from the inkjet printhead(s) secured to the carrier. In some systems, the carrier, and therefore the inkjet printhead(s) secured thereto, remains stationary while the medium being printed is moved. In other systems, the medium remains stationary while the carrier is moved. In still other systems, both the carrier and the medium are moved.
Ink is supplied to each inkjet printhead from an ink reservoir via an ink line. If air becomes trapped in the ink line and flows into the fluid chamber of the inkjet printhead during printing, such air may interfere with the proper ejection of ink from the nozzles of the inkjet printhead. Also, some types of ink include particulates suspended in a fluid and such ink must be kept in motion and/or periodically agitated to prevent the particulates from falling out of suspension.